Previously, thread cutting taps have been limited to rigid devices with radial grooves cut into a threaded shank, forming lands having threads and cutting edges at the intersection of the land and groove.
Some attempt has been made to utilize movable taps for chasing threads or high speed boring and tapping, however, these devices have been complex and costly to manufacture.
A search of the prior art did not disclose any patents that read directly on the claims of the instant invention, however, the following U.S. patents were considered related:
______________________________________ Patent No. Inventor Issue Date ______________________________________ 4,097,180 Kwieraga June 27, 1978 4,090,808 Nannen May 23, 1978 2,616,103 Stecher Nov. 4, 1952 2,284,768 Ramsdell June 2, 1942 ______________________________________
Kwieraga teaches a collapsible tap with a plurality of chaser elements extending axially from a cylindrical body. These elements are spread outwardly by a conical shaped plunger providing the camming surface, so that axial movement of the plunger produces radial movement of the elements. Terminal portions engage grooves in the side of each element and produce a continuous bias permitting axial movement therebetween.
Nannen discloses a two component tapping device allowing a thread cutting member to be inserted into a threaded hole, and a second element inserted as a guide member wedging tightly adjacent to the cutting member. The pair are rotated as a unit to tap or retap the work surface.
Stecher utilizes a plurality of pivotal thread chasers rotatably mounted into extending slots. Each chaser is provided with a thread cutting portion with a taper, preferably be axial inclination, with respect to a centrally located body. The chasers are shifted outwardly by the cam action of a conical member, manually or automatically, into thread cutting engagement with the workpiece.
Ramsdell employs a plurality of fingers having cutting blades extending therefrom. The fingers are in communication with radial slots within the bore of a body. A conical plug is moved, axially forcing the fingers outwardly. While the fingers are expanded by rotating the cone from the front end, it is not possible to make the adjustment by the nut on the bolt by itself requiring access to both ends of the tool. It will be clearly seen that the prior art has attempted to use expanding cutting elements, but has not been able to simply and easily accomplish the adjustment while positioned within an already threaded hole.